Dry etching method and apparatus

ABSTRACT

An accurate dry etching technique is described that employs a flow of neutral radicals and a light beam. A dry etching apparatus  50  employs a neutral radical flow source  20  and a light beam  40  to irradiate a flow of neutral radicals  32 , so that the velocity component of the neutral radicals parallel to the surface of an object to be etched  12  is reduced, and etches anisotropically the object to be etched, while the object  12  is exposed to the radical flow  32  whose parallel velocity component is decreased. The invention reduces the problem of etching parallel to the substrate while etching perpendicular to the substrate to improve anisotropic dry etching without any adverse or damage producing effect to dielectric or semiconductor layers due to ions.

FIELD OF THE INVENTION

[0001] The present invention relates to a dry etching technique, and inparticular to an anisotropic dry etching method that is employed in thesemiconductor field for the delicate processing of thin film layers, andan apparatus therefor.

[0002] BACKGROUND OF THE INVENTION

[0003] In accordance with increases in the storage densities providedfor semiconductor memories, such as DRAMs, the widths of lines in thinfilm layers deposited on semiconductor substrates have become narrower,and at present, methods are being pursued by which to provide lineshaving widths of 0.1 micron or less.

[0004] Concomitanly, there is a demand for an improvement in thepreciseness available with anisotropic dry etching, which is used forthe delicate fabrication of a thin film layer, i.e., an improvement onthe accuracy provided by etching performed perpendicular to thesubstrate, while the occurrence of etching parallel to the substrate isreduced to the minimum possible.

[0005] In response to this demand, for improved etching properties, aconventional anisotropic dry etching technique now employs the force ofan electric field or of a magnetic field to increase the velocity ofcharged particles traveling perpendicular to a substrate containingreactive ions, such as fluorine ions.

[0006] However, with this method, damage incurred to a semiconductorlayer due to the reactive ions is increased in accordance with theincrease in the kinetic energy of the reactive ions, and either a defectwill occur in the layer, or the quality of the layer will be altered.

[0007] To resolve the conventional problems, it is one object of thepresent invention to provide an accurate dry etching technique having noadverse or damage producing effect.

[0008] It is another object of the present invention to provide a newdry etching technique that employs a particle flow and a light beam,especially a flow of neutral radicals and a light beam.

SUMMARY OF THE INVENTION

[0009] To achieve the above objects, according to the present invention,a dry etching method comprises the steps of: preparing an object to beetched; preparing a flow of neutral radicals; irradiating the flow ofneutral radicals with the light beam to impart a change in the velocitycomponent of the neutral radicals parallel to the surface of the objectto be etched; and etching the object to be etched while the object to beetched is exposed to the flow of the neutral radicals whose parallelvelocity component has been altered.

[0010] Further, according to the present invention, a dry etchingapparatus comprises: a vacuum container; an object table provided in thevacuum container to hold thereon an object to be etched; a neutralradical flow source, located at a position opposite the surface of theobject table on which the object to be etched is mounted, for emitting aflow of neutral radicals directed toward the object to be etched mountedon the object table; and a light beam source, located between the objecttable and the neutral radical flow source, for emitting a light beam toirradiate the flow of the neutral radicals emitted by the neutralradical flow source to impart a change in the velocity component of theneutral radicals parallel to the surface of the object to be etched.

[0011] More specifically, a dry etching method comprises the steps of:preparing an object to be etched; preparing a flow of neutral radicals;irradiating the flow of neutral radicals with a light beam so that theirvelocity component parallel to a surface of the object is reduced; andexposing the flow of neutral radicals whose parallel velocity componentis decreased as it approaches the object and upon impact, etching theobject.

[0012] In addition, according to the present invention, a dry etchingapparatus more specifically comprises: a vacuum container; an objecttable provided in the vacuum container on which to mount thereon anobject to be etched; a radical flow source, located at a positionopposite the surface of the object table on which the object to beetched is mounted, for emitting a flow of neutral radicals toward theobject to be etched mounted on the object table; and a light beamsource, located between the object table and the radical flow source,for irradiating a light beam toward the flow of the neutral radicalsemitted by the radical flow source, so that velocity component of theneutral radicals parallel to the surface of the object to be etched isnegligible compared to the velocity component perpendicular to thesurface.

DESCRIPTION OF THE DRAWINGS

[0013] These and other features, objects, and advantages of the presentinvention will become apparent upon consideration of the followingdetailed description of the invention when read in conjunction with thedrawing in which:

[0014]FIG. 1 is a side view of a cross-section of a dry etchingapparatus according to one embodiment of the present invention.

[0015]FIG. 2 is a top view of a cross-section of the dry etchingapparatus according to the embodiment of the present invention.

[0016]FIG. 3 is a flowchart for a dry etching method according to thepresent invention.

[0017]FIG. 4 is a graph showing the relationship between an atomicresonance absorption curve and emission spectra for laser beams.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018]FIG. 1 is a side view of a cross-section of a dry etchingapparatus 50 according to one embodiment of the present invention.

[0019] An object table 14 for mounting thereon an object to be etched12, and an evacuation opening 16 are formed in the lower portion of avacuum container (chamber) 10. An evacuating pipe 18 communicates withthe opening 16, and the end of the pipe 18 is connected to a vacuum pump19, such as a turbo molecular pump.

[0020] A radical flow source (also called a“radical gun”) 20 is locatedat a position above the vacuum container 10 and opposite to the objecttable 14, and emits a flow of neutral radicals toward an object to beetched. An opening 22 is formed for the introduction by the radical flowsource 20 of F₂, SF₆, NF₃, CF₄ or other halogenated chemicals, which aregases for the production of neutral halogen radicals which may be asingle halogen atom, a free radical containing a halogen atom or one ormore atoms forming a molecule or molecular fragment containing at leastone atom of a halogen. A free radical may be a group of atoms bondedtogether that is considered an entity in various kinds of reactions. Inaddition, electrodes 23 are provided in the radical flow source 20. AnRF/DC power source 24 is connected to the electrodes 23 to performdecomposition of the gas and separation of ions and radicals between theelectrodes 23. A microwave source may be employed instead of the RF/DCpower source 24 to decompose the gas. The radical flow source 20 has anopening 26 and a guide 28, which is so connected to the vacuum container10 that it covers an opening 30 formed in the vacuum container 10.

[0021] A transparent window 34 is formed in a side wall of the vacuumcontainer 10, and a light beam source 36 is located at a positionopposite the window 34. A laser beam source is used as the light beamsource 36. A mirror 38 is provided on the side wall of the vacuumcontainer 10 opposite the window 34.

[0022]FIG. 2 is a cross-sectional top view of the center portion of thedry etching apparatus 50. Two light beam sources are provided: the lightbeam source 36 in the X axial direction and the light beam source 36′ inthe Y axial direction. The window 34 and the mirror 38 are provided forthe light beam source 36, while the window 34′ and the mirror 38′ areprovided for the light beam source 36′.

[0023]FIG. 3 is a flowchart showing a dry etching method according tothe present invention. Step (A) Preparation of object to be etched isshown in box 39 in FIG. 3. An object to be etched is mounted on theobject table 14 of the vacuum container 10, and then air in the vacuumcontainer 10 is evacuated by a vacuum pump 19, such as a diffusion pumpor a turbo molecular pump, until a vacuum is attained in the container10. The object to be etched is an Si substrate on which is depositedSiO₂, for which, for example, a pattern photoresist layer is formed.

[0024] Step (B), Preparation of radical flow is shown in box 40 in FIG.3. Gas for a radical flow, such as F₂, SF₆, NF₃ or CF₄, is introducedinto the neutral radical flow source 20 through the opening 22. The gasis decomposed between the electrodes 23 by the RF power supplied fromthe RF/DC power source 24. Ions and fluorine radicals are separated bythe electric field generated by a DC voltage, and only the fluorineradicals are introduced through the opening 26. The flow of radicals,after being passed through the opening 26, is transmitted along theguide 28 and enters the vacuum container 10 through the opening 30.

[0025] At this time, because of a difference in pressure between theradical flow source 20 (the guide 28) and the vacuum container 10, thefluorine radical flow is linearly directed as a beam toward the objectto be etched. It should be noted, however, that as is represented by 32in FIG. 1, the fluorine radical beam spreads as it approaches the objectto be etched. The internal (excitation) energy and the velocity of thefluorine radicals are controlled by gas pressure, RF power, a DC voltageand the distance between the electrodes.

[0026] Step (C), Adjustment of velocity of radical flow is shown in box41 in FIG. 3. The fluorine radical flow, which linearly transits thevacuum container 10 toward the object to be etched, is irradiated bylight beams 40 in the X and the Y axial directions that respectively areemitted by the light beam sources 36 and 36′. The light beams 40 areemitted in directions perpendicular to the direction in which thefluorine radical beam is projected. The light beams 40, afterirradiating the fluorine radical beam, are reflected by the mirrors 38and 38′, and again irradiate the fluorine radical beam.

[0027] A laser beam source is used as the light beam source 36 forirradiating the fluorine radicals. The laser that is employed has

[0028] a central emission wavelength of “685.6+Δ” nm, which is slightlylonger than the 685.6 nm that is a resonance wavelength for transientenergy between the 3 s⁴P(5/2) level and the 3 p⁴D⁰(7/2) level offluorine atoms. This is because the Doppler cooling effect, which willbe described later, is employed. “Δ” indicates an appropriate wavelengthwidth that is smaller than 1 nm.

[0029] When a laser beam irradiates the flow of radicals, the velocitycomponent of the radicals parallel to the surface of the object to beetched is reduced due to the Doppler cooling effect. That is, sincekinetic momentum is exchanged by the photons of the laser beam emittedby the laser beam source 36 and fluorine atoms, the velocity componentof the fluorine atoms in the X axial direction (FIG. 2) is reduced.Similarly, since kinetic momentum is exchanged between the photons ofthe laser beam emitted by the laser beam source 36′ and fluorine atoms,the velocity of the fluorine atoms in the Y axial direction (FIG. 2) isreduced. As a result, the velocity component perpendicular to thesurface of the object to be etched (in the Z direction) is relativelyincreased. It is possible to reduce the velocity components of thefluorine atoms in the X and Y axial directions almost to zero, and as aresult, the fluorine radicals will enter or impact the surface of theobject to be etched perpendicularly. Therefore, the control of the locusof neutral radicals, which is difficult conventionally, is implemented.

[0030] The Doppler cooling effect will be further explained. The Dopplercooling effect is attained by the employment of the optical Dopplereffect to reduce the velocity of moving neutral atoms. A detailedexplanation on will be given below.

[0031]FIG. 4 is a graph showing a relationship between an atomicresonance absorption curve 42 and the emission spectra of laser beams. Aresonance absorption curve 42 in FIG. 4 peaks at central frequency f₀,and represents the relationship between the resonance absorption ratefor fluorine atoms and the frequency of light. Lines 43, 44 and 45represent emission spectra for laser beams having their central emissionfrequencies f₁, f₂ and f₃ as seen by a fluorine atom. Specifically, thecentral frequency f₀ corresponds to 685.6 nm, which is the resonancewavelength for transition energy between the 3s⁴P(5/2) level and the3p⁴D⁰(7/2) level for fluorine atoms. In addition, this frequency islower than the frequency f₀ of the laser beam the equivalent of theabove described wavelength width Δ.

[0032] Assume that the fluorine atoms are moving at velocity v towardthe laser beam (toward the laser beam source 36 in FIG. 2) that isemitted by the laser beam source 36 and that along the X axis has afrequency of f₁. In this case, when the light velocity is c, thefrequency f₂ for the laser beam viewed from the fluorine atoms is

f ₂ =f ₁/(1−v/c).

[0033] That is, the emission spectrum for the laser beam viewed from thefluorine atoms shows a curve 44, as in FIG. 4, not a curve 43, in FIG.4.

[0034] Assume that the fluorine atoms are moving at velocity v in thesame direction as the laser beam (in the direction toward the mirror 38along the X axis). The frequency f₃ of the laser beam viewed from thefluorine atoms is

f ₃ =f ₁/(1+v/c).

[0035] Then, the emission spectrum shows a curve 45, as in FIG. 4, notthe curve 43.

[0036] Let us discuss the relationship between the resonance absorptioncurve and the change in the frequency of the light, relative to thefluorine atoms, that is due to the Doppler effect. As is apparent fromFIG. 4, since the frequency is nearer the peak resonance absorptionfrequency when it is changed to the frequency f₂ (spectrum 44), morelight resonance absorption occurs than when the frequency is changed tothe frequency f₃ (spectrum 45). That is, light resonance absorption forthe fluorine atoms that are moving toward the laser beam source 36 alongthe X axis in FIG. 2 is greater than that for the fluorine atoms thatare moving toward the mirror 38 on the opposite side, and as a result,there is a difference in the quantities of light they absorb.

[0037] The fluorine atoms, which are moving toward the light source 36that has absorbed more light, are altered and enter an excited state,i.e., their energy level is changed to the excitation level. Then, thelight is emitted and the fluorine atoms acquire a more stable energystate, i.e., they are returned to the lower energy level. Since thelight emitted at this time is isotropically radiated by the atoms, noforce is applied to the fluorine atoms in a specific direction due tothe spontaneous emission of the light.

[0038] Under a situation of momentum conservation, the fluorine atomsmoving toward the laser beam source 36 absorb not only the light butalso are influenced by the kinetic momentum held by photons. The kineticmomentum influencing the fluorine atoms acts as force to move thefluorine atoms in a direction opposite to that in which they aretraveling. That is, under the conditions associated with moumentumconversation, an exchange of kinetic momentum occurs between the atomsand the photons, and the fluorine atoms are subjected to a driving forceimpelling them toward the mirror 38, which is in the opposite directionto the direction in which they are traveling along the X axis. As aresult, the velocity of the fluorine atoms, which are traveling alongthe X axis toward the laser beam source 36, is reduced.

[0039] the fluorine atoms that are traveling toward the mirror 38 alsoacquire the kinetic momentum from the photons. However, since thesefluorine atoms do not absorb much light, the acquired momentum isaccordingly negligible. As a result, no force is applied, in thedirection of travel, to the fluorine atoms that are traveling toward themirror 38. The velocity increase is also negligible.

[0040] The same conditions can be predicated for the fluorine radicalatoms that are traveling in concert with a laser beam that is reflectedby the mirror 38. More specifically, the velocity of the fluorineradical atoms that are traveling toward the X axial laser beam (towardthe mirror 38) can be reduced by the Doppler cooling effect. In additionto the previously described reduction in the velocity when the fluorineatoms are traveling toward the X axial laser beam 36, the velocity ofthe fluorine atoms is reduced in the X axial direction.

[0041] The above explanation is given for a case wherein fluorine atomsare traveling along the X axis. Actually, the X axial component v_(x) ofthe velocity vector v of the fluorine atoms is reduced.

[0042] The same conditions can be predicated for the Y axis in FIG. 2.The velocity of the fluorine radical atoms traveling in the Y axialdirection can be reduced by the Doppler cooling effect produced by alaser beam. As a result, the velocity component, of the fluorine radicalatoms that are irradiated by the laser beam, that is perpendicular tothe surface of the object to be etched (in the Z axial direction) isrelatively increased.

[0043] Step (D), Dry etching is shown in box 47 in FIG. 3.

[0044] The object to be etched is etched in a direction perpendicular tothe surface by reactions occurring between the fluorine radicals, whichare perpendicularly incident to the surface of the object, and the atomsor molecules of the object. In short, the object to be etched isanisotropically etched by the fluorine radicals.

[0045] When a Si substrate is employed on which is deposited SiO₂, forwhich a patterned photoresist layer is provided, a predetermined SiO₂pattern is formed by dry etching as outlined in FIG. 3. No damage due toions is introduced in a pattern formed on the SiO₂ or the Si substrate.Since the fluorine radicals are perpendicularly incident to the surfaceof the object to be etched without having any velocity componentsparallel to the surface, a more delicate anisotropic pattern can beformed.

[0046] The preferred embodiment has been described, but the presentinvention is not limited to this embodiment and can be variouslymodified or applied as within the scope of the subject of the presentinvention. In the above embodiment, the light beam is used to reduce thevelocity component of the fluorine radicals traveling parallel to thesurface of the object to be etched; however, the light beam may be usedto increase the above velocity component and to relatively reduce thevelocity component perpendicular to the surface of the object (in the Zaxial direction), so that a flow of fluorine radicals can be spreadacross the surface of the object to be etched. For example, f₁, may beabove f₀ instead of below f₀, by the same absolute amount to utilizecurve 42 above f₀ to increase the parallel velocity component.

Having thus described our invention, what we claim as new and desire tosecure by letters patent is:
 1. A method for dry etching comprising thesteps of: preparing an object to be etched; preparing a flow of neutralradicals; irradiating said flow of neutral radicals with a light beam sothat their velocity component parallel to a surface of said object isreduced; and exposing said flow of neutral radicals whose parallelvelocity component is decreased to said object and etching said object.2. The dry etching method according to claim 1, wherein said step ofreducing said parallel velocity component of said neutral radicalsincludes a step of exchanging kinetic momentum between the photons ofsaid light beam and said neutral radicals (Doppler cooling effect) toreduce said velocity component of said neutral radicals parallel to saidsurface of said object to be etched.
 3. The dry etching method accordingto claim 2, wherein said step of preparing said object to be etchedincludes a step of mounting said object to be etched on a table providedin a vacuum container.
 4. The dry etching method according to claim 3,wherein said step of preparing said neutral radicals includes a step ofemitting a neutral radical beam toward said object to be etched from asource of the neutral radical flow provided at a position opposite to anetching surface of said object to be etched mounted on said table. 5.The dry etching method according to claim 4, wherein said step ofirradiation with said light beam includes irradiation with a light beamtraveling in a direction that is substantially perpendicular to adirection in which said radical beam is projected.
 6. The dry etchingmethod according to claim 4, wherein said step of irradiation with saidlight beam includes irradiation with a light beam traveling along pathsleading in at least two different directions that are substantiallyperpendicular to a direction in which said radical beam is projected. 7.The dry etching method according to claim 6, wherein said two differentdirections diverge at an angle of 90 degrees.
 8. The dry etching methodaccording to claim 2, wherein said light beam has a wavelength of “W+Δ,” which is slightly greater than a wavelength of “W” that resonates whensaid neutral radicals have a predetermined transition level.
 9. The dryetching method according to claim 1, wherein said neutral radicalsinclude radical fluorine atoms.
 10. The dry etching method according toclaim 1, wherein said light beam is a laser beam.
 11. The dry etchingmethod according to claim 1, wherein said object to be etched includes asemiconductor substrate.
 12. The dry etching method according to claim9, wherein said laser beam has a light wavelength of 685.6+Δ [nm], whenradical fluorine atoms are used.
 13. A dry etching apparatus comprising:a vacuum container; an object table provided in said vacuum container onwhich to mount thereon an object to be etched; a radical flow source,located at a position opposite the surface of said object table on whichsaid object to be etched is mounted, for emitting a flow of neutralradicals toward said object to be etched mounted on said object table;and a light beam source, located between said object table and saidradical flow source, for irradiating a light beam toward said flow ofsaid neutral radicals emitted by said radical flow source, so thatvelocity component of said neutral radicals parallel to the surface ofsaid object to be etched is negligible compared to the velocitycomponent perpendicular to said surface.
 14. The dry etching apparatusaccording to claim 13, wherein a source of said radical flow isconnected to said vacuum container so that a beam-shaped flow of neutralradicals is introduced to said vacuum container through an opening thatis formed in said vacuum container.
 15. The dry etching apparatusaccording to claim 14, wherein said light beam source is connected tosaid vacuum container so that a light beam is introduced into saidvacuum container through a transparent window that is formed in a sidewall of said vacuum container.
 16. The dry etching apparatus accordingto claim 15, wherein said light beam source is so located that saidlight beam source emits a light beam traveling along a path in adirection that is substantially perpendicular to a direction in whichsaid radical beam flow is projected.
 17. The dry etching apparatusaccording to claim 15, wherein said light beam source is so located thatsaid light beam source emits a light beam traveling along paths leadingin at least two different directions that are substantiallyperpendicular to a direction in which said radical beam flow isprojected.
 18. The dry etching apparatus according to claim 17, whereinsaid two different directions are away from each other almost at 90degrees.
 19. The dry etching apparatus according to claim 15, furthercomprising a mirror, provided inside or outside of a side wall of saidvacuum container opposite to said side wall whereat said transparentwindow is formed, for reflecting a light beam from said light beamsource to again irradiate said flow of radicals.
 20. The dry etchingapparatus according to claim 13, wherein said velocity component of saidradicals parallel to said surface of said object to be etched is reducedas a result of a kinetic momentum exchange between the photons of saidirradiated light beam and said neutral radicals (Doppler coolingeffect).
 21. The dry etching apparatus according to claim 20, whereinsaid light beam has a wavelength of “W+Δ, ” which is slightly greaterthan a wavelength of “W” that resonates when said neutral radicals havea predetermined transition level.
 22. The dry etching apparatusaccording to claim 21, wherein said neutral radicals include radicalfluorine atoms or other halogen radicals.
 23. The dry etching apparatusaccording to one of claims 13 to 22, wherein said light beam source is alaser beam source.
 24. The dry etching apparatus according to claim 22,wherein said laser beam has an emission wavelength of 685.6+Δ [nm], whenradical fluorine atoms are used.
 25. A dry etching method comprising thesteps of: preparing an object to be etched; preparing a flow of neutralradicals; irradiating said flow of neutral radicals with a light beam toimpart a change in the velocity component of said neutral radicalsparallel to the surface of said object to be etched; and etching saidobject to be etched while said object to be etched is exposed to saidflow of said neutral radicals whose parallel velocity component has beenaltered.
 26. A dry etching apparatus comprising: a vacuum container; anobject table provided in said vacuum container to hold thereon an objectto be etched; a neutral radical flow source, located at a positionopposite the surface of said object table on which said object to beetched is mounted, for emitting a flow of neutral radicals directedtoward said object to be etched mounted on said object table; and alight beam source, located between said object table and said neutralradicals flow source, for emitting a light beam to irradiate said flowof said neutral radicals emitted by said neutral radical flow source toimpart a change in the velocity component of said neutral radicalsparallel to the surface of said object to be etched.